linux-kernel - Re: [PATCH 2/2] dt-bindings: i2c: arb-gpio-challange: convert to DT schema

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Message-ID: <20230722-overlay-molehill-4213d2143609@spud>
Date:   Sat, 22 Jul 2023 13:07:08 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Peter Rosin <peda@...ntia.se>, Andi Shyti <andi.shyti@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Doug Anderson <dianders@...omium.org>,
        linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] dt-bindings: i2c: arb-gpio-challange: convert to DT
 schema

On Sat, Jul 22, 2023 at 11:57:10AM +0200, Krzysztof Kozlowski wrote:

> +description: |
> +  This uses GPIO lines and a challenge & response mechanism to arbitrate who is
> +  the master of an I2C bus in a multimaster situation.
> +
> +  In many cases using GPIOs to arbitrate is not needed and a design can use the
> +  standard I2C multi-master rules.  Using GPIOs is generally useful in the case
> +  where there is a device on the bus that has errata and/or bugs that makes
> +  standard multimaster mode not feasible.
> +
> +  Note that this scheme works well enough but has some downsides:
> +   * It is nonstandard (not using standard I2C multimaster)
> +   * Having two masters on a bus in general makes it relatively hard to debug
> +     problems (hard to tell if i2c issues were caused by one master, another,
> +     or some device on the bus).
> +
> +  Algorithm:
> +  All masters on the bus have a 'bus claim' line which is an output that the
> +  others can see. These are all active low with pull-ups enabled.  We'll
> +  describe these lines as:
> +   * OUR_CLAIM: output from us signaling to other hosts that we want the bus
> +   * THEIR_CLAIMS: output from others signaling that they want the bus
> +
> +  The basic algorithm is to assert your line when you want the bus, then make
> +  sure that the other side doesn't want it also.  A detailed explanation is
> +  best done with an example.
> +
> +  Let's say we want to claim the bus.  We:
> +  1. Assert OUR_CLAIM.
> +  2. Waits a little bit for the other sides to notice (slew time, say 10
> +     microseconds).
> +  3. Check THEIR_CLAIMS.  If none are asserted then the we have the bus and we
> +     are done.
> +  4. Otherwise, wait for a few milliseconds and see if THEIR_CLAIMS are released.
> +  5. If not, back off, release the claim and wait for a few more milliseconds.
> +  6. Go back to 1 (until retry time has expired).

> +  their-claim-gpios:
> +    minItems: 1
> +    maxItems: 2
> +    description:
> +      The GPIOs that the other sides use to claim the bus.  Note that some
> +      implementations may only support a single other master.

Where does the maxItems: 2 come from?

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